Modern internal combustion engines can be operated in different combustion modes. One example to mention is a change between a stoichiometric spark-ignited homogenous mode and a lean stratified-charge mode or the controlled auto-ignition method (CAI method). The latter is characterized by the fact that a lean homogeneous air-fuel mixture is brought to auto-ignition in a controlled way without producing significant NOx emissions at the same time. The ignition is initiated by hot exhaust gas retained in the cylinder and the increase in pressure and temperature during the compression phase. It is obvious that the fuel quality plays an important role in the respective combustion method. This relates in general to the position and size of the operating range of a combustion method, in particular the combustion control and hence the fuel consumption and the engine's emission behavior.
Known from the prior art, such as that disclosed in U.S. Pat. No. 7,073,466 is a method for regulating a combustion process of an HCCI internal combustion engine. Hereby, the internal combustion engine can be operated at least in certain operating modes with controlled auto-ignition (HCCI mode). In controlled auto-ignition, an actual combustion process and a modeled combustion process are continuously compared to each other with the difference between output variables of the actual combustion process being referred to the modeled combustion process and traced according to this process.